The public relations hype news about the Chevy Volt possibly getting a 100 mpg rating from either the EPA or the California Air Resources Board could go down as one of the better attempts by a car maker to dupe consumers. The GM marketing machine seems to be trying to change the rules of the game to fit its skewed own model.

According to General Motors E-Flex spokesman Rob Peterson, the automaker has reached an agreement with the California Air Resources Board (CARB) that would see the 2011 Chevy Volt get a unique classification different from other current hybrids. This new classification takes into account the fact that the Volt’s 40-mile battery range allows it to complete the bulk of the emissions and economy test procedure without ever running the engine, which would likely give it a mpg rating of 100 mpg or better. [On AutoBog.com]

FYI, the EPA hybrid testing cycle currently requires that dual-power vehicles “…complete the test cycle with a charged battery.” This dooms the Volt to an overall 48 mpg rating — Prius and Civic hybrid territory.

According to GM, the Volt doesn’t use any fuel for the first 40 miles of driving, but having to end the cycle with a full battery will require the use of gasoline for recharging. So, based on GM’s argument, which implies thinking that the Volt shouldn’t have to end the testing cycle with a full battery, drivers shouldn’t be concerned with the second 40 miles, during which the Volt’s mpg will fall.

Assuming the Volt gets 50 mpg with the gasoline engine running to charge the battery, its actual mpg rating should require a sliding scale. 

Taking into account that the car can go 40 miles using no gas, if one where to drive 50 miles, during the last 10 miles it would use about 0.2 gallons, equaling 250 mpg.

While the average commute for U.S. workers nears 50 miles a day, add any additional miles — pick up the kids, groceries and that coffee iced double tall soy latte — and the rule of diminishing returns comes into play. Add another 30 miles for a total of 80 miles (40 on battery/40 with the engine running) and the total fuel economy falls to about 100 mpg.

Visit the relatives 300 miles away, and the Volt’s mpg dips to 62.5.

This is without including the cost of the electricity to charge the Volt when plugged in. And assuming 50 mpg with the engine running. And assuming driving only on flat ground. (I know from experience that the Prius mpg drops significantly going up hills.)

Does this mean an electric-only vehicle, which doesn’t have a gasoline tank, get ∞ miles per gallon?

Perhaps the EPA should determine mileage over a testing regime that runs for the equivalent of a tank of gasoline, say 400 miles?  Sort of like us consumers do?

The truth is somewhere in between, to be sure.